SOL-GEL SYNTHESIS OF SIO2-P2O5 GLASSES

The synthesis of P2O5-SiO2 glasses from solutions has been studied through gelation kinetics, Raman and nuclear magnetic resonance (NMR) spectroscopies. Different preparation methods are compared. The effect of the phosphorus precursor has been investigated. Phosphoric acid esters are not hydrolyzed and they do not yield condensation with silica. Reaction of anhydrous H3PO4 with Si-OR groups leads to the formation of Si-O-P bonds which are easily hydrolyzed. Only a few remain after addition of water and gelation. Therefore, P-Si homogeneity is difficult to achieve after gelation and the following drying and heat-treatment procedures. This absence of homogeneity is observed by solid-state magic angle spinning (MAS)-NMR and X-ray diffraction studies performed on gel samples heated at 300, 500 and 800 degrees C in the case of xerogels and 500, 950 and 1140 degrees C for aerogels.